A conventional range hood scarcely differs from a ceiling-mounted range hood in a working space. The flange of this kind of range hood may be in a flat shape, a downward arc shape, or in a box-shape, and the air-extraction opening may be in a round shape, a square shape, a rectangular shape, or in an elongate shape. When this kind of range hood draws air, the air and pollutants would flow into the space below the hood from the edge of the flange, then be drawn into the air-extraction opening, and afterwards the air and pollutants would be exhausted outside through ducts. In this case, the air flow rate at the location near the air-extraction opening is high, while the air flow rate at the location away from the air-extraction opening decreases rapidly with the increase of the distance from the air-extraction slot. The upward force of extraction would be insufficient because the upward air velocity becomes low within a short distance below the air-extraction opening so that the flow field is easily affected by drafts from people walking, fans or air conditioner operating. Under such interferences, the soot and pollutants would disperse by the interference flow.
When above range hood is used in high-temperature production or cooking processes, the speed of updraft would increase due to the large buoyant effect resulted from high temperature of fire. Under these conditions, the soot and pollutants would disperse out easily from the front, the rear, and the lateral sides of the range hood due to turbulent diffusion and expansion effect caused by the high temperature of fire. Therefore, the conventional soot-exhausting device would enhance the force of extraction to improve the extraction capacity. However, in this way, not only does the noise increase and the energy waste, but the leakage also could not be prevented completely.
As for above problems, a common improvement is shown in FIG. 8, where baffles were added respectively in the left side, right side, and the rear of the soot-exhausting device (a). However, when in implementation, the boundary layers of flow are easily departed at the front edge of both side plates (b) so as to produce a large recirculation bubble (c) respectively. Soot pollutants would be entrained and carried by the flow of this big recirculation bubble (c) to the front edge of both side plates (b) nearby and be leaked out as a result of molecular diffusion and turbulent diffusion or be swept to external environment as a result of the effect of environment interference flow. Complicated three dimension vortexes and strong turbulences would be produced at the cross of the flange (d) and the side plate (b) of the soot-exhausting device (a) as a result of the three-dimensional effect. In this case, soot pollutants would also be swept to the cross of the flange (d) and the side plate (b) of the soot-exhausting device (a) to cause leakage. For this reason, it is unable to achieve the effect of preventing leakage completely simply by adding side plates (b) respectively in the left and right side of the soot-exhausting device (a).